This invention relates to a method for panoramic dental X-raying, in which the beam of rays emitted from a radiation source mounted on a rotating arm is guided through the dental arch to a recorder located in the arm opposite the radiation source in order to form an image, and in which the arm is rotated so as to form an image of essentially the entire length of the dental arch. The invention also relates to an apparatus for implementing said method.
Panoramic X-raying aims at forming images of the teeth in a projection that is as orthogonal, i.e. perpendicular as possible, thus avoiding having the teeth imaged in an overlapping manner. Since the dental arch differs in shape from a circular arch, the axis of rotation of the arm must be shifted in course of the imaging in order to achieve the purpose of orthogonal imaging. The radius of curvature of the dental arch being smallest in the front area of the arch, it is preferred to approach the axis of rotation to the teeth when this area is imaged in order to minimise the need for shifting the axis. During X-raying of the molar teeth, the distance between the axis and the spot of the teeth to be imaged is typically longer.
Due to the short distance between the axis of rotation of the arm and the spot of the teeth to be imaged in the front area of the dental arch, the sharply imaged teeth layer will be narrow in this area. In terms of imaging geometry, the spots located in a single plane through which the beam of rays penetrates each time will be projected as spots to the recorder. Beyond this plane, the spots covered by the beam of rays will be projected as elongated lines to the recorder, and the thickness of the xe2x80x9csharpxe2x80x9d layer on either side of the plane is determined by the degree of extension considered acceptable. Generally a 0.6 mm blur is used as the limit of the sharp layer, blur implying the spot extension divided by the ratio of enlargement.
A narrow sharp layer, i.e. a thin layer in the front area of the dental arch, involves a drawback, since the teeth position divergence is highest precisely in this area. If the teeth are not included in the sharp layer, but will be located more or less outside this, the X-raying has failed. There is thus a need to compensate for this drawback so as to achieve increased probability of successful images and to reduce the need for repeated X-raying sessions, which would expose the patient to additional radiation.
The object of the invention is thus to provide a solution for eliminating the drawback of current panoramic X-raying techniques mentioned above. The invention comprises limiting the X-ray beam by a shutter in the front area of the dental arch as compared to the two sides of the arch, in order to increase in said front area the thickness of the layer of which a sharp image is formed, and retarding the movement of the X-ray beam in said front area of the dental arch compared to the two sides of the arch.
The invention is based on imaging geometry, in which the thickness of a sharply imaged layer is inversely proportional to the width of the beam of rays on the recorder. Should the beam be infinitely narrow, it would form an infinitely thick image, in other words, all the objects located between the source of radiation and the recorder would be imaged with equal sharpness. Inversely, the broader the beam of rays, the narrower the sharply imaged layer between the source of radiation and the recorder. In the practice, the beam of rays obtained from the source of radiation has a finite width, and by adjusting this width under control with the shutter in accordance with the invention, the invention allows compensation for the decreased thickness of the sharp layer that would occur in the front area of the dental arch if no adjustment were performed.
In accordance with the invention, the increased thickness of the sharp layer in the front area of the dental arch facilitates the positioning of the patient, provides higher probability of successful images and reduces the exposure to radiation of the patients. It is particularly notable that the increased layer thickness provides enhanced visibility of the dental structures, such as the root tips, in the pictures.
An adjustable shutter of X-ray beams is not novel per se, but has been disclosed i.a. by FI lay-out print 64999 (Patent Application 802,988) and FI Patent Application 812,954. However, these references do not describe the use of a shutter with a view to decrease to thickness of the sharp layer in the front area of the teeth, as in the present invention.
As it has appeared above, there is a need for compensating for the narrowed sharp layer in the front area of the dental arch especially in such panoramic X-raying applications where the axis of rotation of the arm is shifted during the imaging session so that the distance between the axis and the imaged spot of the dental arch, i.e. the xe2x80x9cradius of imagingxe2x80x9d is shortest in the front area of the arch. In this area, the X-ray beam penetrates the dental arch substantially perpendicularly. The solution to this is to limit the X-ray beam as in the invention on either side of the central normal to the arch in a sector with a central angle of e.g. approx. 60-80xc2x0, preferably about 70xc2x0.
In accordance with the invention, the X-ray beam is narrowed in the front area of the dental arch preferably so as to obtain an at least 50% increase of the thickness of the sharp layer. The layer thickness of the sharp layer will preferably be about 1.5 cm or more in the front area of the arch. With current panoramic X-raying methods, involving a short imaging radius in the front area of the dental arch, the thickness of the sharp layer is typically of the order of magnitude of about 1 cm or even less, without the compensation of the invention.
Since a narrower X-ray beam decreases the radiation impinging on the recorder, in order to achieve adequate exposure, it is necessary to retard the movement of the X-ray beam in the front area of the dental arch relative to the two sides of the arch. In this connection, the movement of the beam refers to the speed. The beam is moved or scanned along the dental arch in a direction transverse to the radiation direction. In the practice, the retardation can be of the order of about 40%-60%, preferably about 50%. By these means, a substantially homogeneous dark tone of the image is available over the entire length of the dental arch.
Since, in the practice, it is desirable to X-ray the dental arch with one single continuous rotational movement, the X-ray beam is limited in accordance with the invention as the movement reaches the front area of the dental arch, and after this the beam is accordingly enlarged to its original width as it leaves this area. It is thus preferable to connect the adjustment of the shutter with the movement performed during the imaging, for instance by narrowing and widening the shutter aperture through which the X-ray beam passes under mechanical control of the movement of the axis of rotation of the arm. Retardation and subsequent acceleration of the movement of the X-ray beam would take place in tandem with the gradual narrowing and widening of the shutter to ensure a substantially constant exposure of the arch to X-rays through the entire length of the arch.
The apparatus of the invention for panoramic dental X-raying, which comprises an arm rotating about an axis, a radiation source at one end of the arm for emitting an X-ray beam, a shutter shaping the X-ray beam, and a recorder placed at the opposite end of the arm to receive the X-ray beam after it has passed through the dental arch for forming an image of the dental arch, is characterised by the shutter comprising an aperture through which the X-ray beam passes and which is disposed to decrease in width during the rotational movement of the arm and to subsequently resume its original width. The adjustment of this aperture is preferably performed under mechanical control by the arm rotating mechanism.